Field of the Invention
The present invention relates to a method of manufacturing a cylindrical bonded magnet and a manufacturing equipment for the cylindrical bonded magnet.
Discussion of the Background
A bonded magnet composed of a magnetic material and a resin that serves as a binder for the magnetic material can be produced in more complex shapes than a sintered magnet, and also has superior mechanical strength. Therefore, bonded magnets are widely used as electronic parts in permanent magnet-type synchronized motors (DC motors and stepping motors), in laser printer magnet rolls, and so forth.
Methods for manufacturing such bonded magnets can be broadly classified into three types: injection molding, compression molding, and extrusion molding.
Of these manufacturing methods, injection molding involves heating a bonded magnet composition composed of a magnetic material and a thermoplastic resin in the cylinder of an injection molding machine to put the composition into a molten and fluid state, using a plunger to fill the interior of a metal mold, and molding the composition into the desired shape.
Compression molding involves filling a press mold with a bonded magnet composition composed of a magnetic material and a thermosetting resin, and molding under compression.
In the steps of the above-mentioned compression and injection molding methods, there is a set cycle composed of filling a mold with a bonded magnet composition, molding, and taking out the bonded magnet (the molded article), and because this involves what is known as batch production, there is a limit to the production speed.
Also, in injection molding and compression molding, there are limits in terms of molding slender articles such as elongated molded products. One reason is the problem of machining the mold. The shape of the molded article is cut into the mold, but high-precision machining in the depth direction of a mold is extremely difficult. Another reason is a problem in molding. In the case of compression molding, when a slender object is pressed, the pressure is not transmitted to the middle of the molded article. Also, when a slender article is molded by injection molding, the bonded magnet composition ends up cooling down after going through the gate, resulting in a short shot (improper molding due to incomplete filling with the molding material).
In contrast to the above methods, extrusion molding involves heating a bonded magnet composition composed of a magnetic material and a thermoplastic resin or thermosetting resin in a cylinder to melt the composition and put it in a fluid state, and continuously supplying this fluid-state bonded magnet composition to a mold to mold it into the desired shape. Therefore, extrusion molding is a continuous process, unlike the batch process of injection or compression molding, so productivity is much higher. Furthermore, since molding can be performed continuously, the molding of slender articles, which was difficult with injection molding and compression molding, can be accomplished with ease.
The magnetic material that makes up a bonded magnet will now be discussed. The raw material composition of this magnetic material can be classified into magnetic materials that are ferrite based and rare earth based. Ferrites have a long history and are inexpensive, which makes them more popular. Ferrites, however, have weaker magnetism than rare earths, and their magnetism may not be strong enough if the molded article is small. Therefore, with small molded articles it is preferable to use a rare earth-based magnetic material.
Also, from the standpoint of the mechanism by which magnetism is exhibited, the magnetic material that makes up the bonded magnet can be classified into isotropic and anisotropic. Isotropic magnetic materials exhibit the same magnetic force in every direction, whereas anisotropic magnetic materials can exhibit a strong magnetic force in only one direction. Therefore, when an anisotropic magnetic material is made into a bonded magnet, the direction of magnetization of the particles of the magnetic material has to be aligned in a specific orientation to effect anisotropization. This operation is called orientation. This orientation can be broadly broken down into two types: mechanical orientation and magnetic field orientation. “Mechanical orientation” makes use of the fact that when a magnetic material is made up of flat particles, the flat particles align in their thickness direction when a pressure is applied externally to the flat particles during molding. If the flat particles have an axis of easy magnetization in their thickness direction, then the particles of the magnetic material can be mechanically oriented by this operation. “Magnetic field orientation,” meanwhile, refers to orienting particles by applying a magnetic field externally during molding. With a ferrite-based magnetic material, mechanical orientation is also possible due to the relation between particle shape and the direction of the axis of easy magnetization, but with a rare earth-based magnetic material, only magnetic field orientation is possible. When an anisotropic magnetic material is used, orientation entails more steps than when an isotropic magnetic material is used, so molding becomes more difficult, but on the other hand the magnetic force is stronger than when an isotropic magnetic material is used.
Cylindrical magnets, which emit magnetic force in the inner circumference direction, are widely used in spindle motors that are installed in hard disk drives and optical media. Nearly all of these are cylindrical bonded magnets obtained by compression molding a bonded magnet composition composed of an isotropic Nd—Fe—B-based magnetic material and a resin. The reason for this is that, as discussed above, with an isotropic Nd—Fe—B-based magnetic material there is no need for a step of orienting the magnetic material during molding, so the molding is extremely simple, and the desired surface magnetic flux waveform can be imparted with just a magnetization step.
However, the only way to increase the surface magnetic flux density of the molded cylindrical bonded magnet is to squeeze in a large quantity of Nd—Fe—B magnetic material per unit volume, and this creates a problem in that the specific gravity of the molded cylindrical bonded magnet ends up being high.
To make a spindle motor smaller and lighter, the cylindrical bonded magnet needs to be even more lightweight and have a higher magnetic force. Therefore, a great deal of research has been conducted into the manufacture of cylindrical bonded magnets by injection molding or compression molding.
Nevertheless, the popularity of anisotropic bonded magnets lags behind that of bonded magnets made with anisotropic Nd—Fe—B-based magnetic materials. The reason for this will be described below.
Let us consider a case in which a cylindrical bonded magnet that emits magnetic force in the inner circumference direction is molded using as the material a bonded magnet composition containing an anisotropic magnetic material. First, in the manufacturing equipment for a bonded magnet, an orientation-use permanent magnet should be disposed in the mold that molds the inner peripheral face of the cylindrical bonded magnet (the molded article).
However, if the molded article is small in size, the size of the individual magnets that make up of the orientation magnet will also be small, so no matter how powerful a permanent magnet is used as a material, the orienting magnetic field that is generated will be weak.
Since it is difficult to orient the magnetic material included in the bonded magnet composition in an orienting magnetic field with a low surface magnetic flux density such as this, the surface magnetic flux density of the cylindrical bonded magnet ends up being low. For example, while the surface magnetic flux density of a cylindrical bonded magnet produced by compression molding using an isotropic Nd—Fe—B-based magnetic material is approximately 2000 G, the surface magnetic flux density of a cylindrical bonded magnet that is a molded article produced by injection molding in a small orienting magnetic field such as this is only approximately 1500 G.
The use of magnets aligned so as to repel each other at outer peripheries in the circumferential direction as an orientation magnet has been studied in an effort to solve this problem of smaller surface magnetic flux density, as disclosed in Japanese Laid-Open Patent Application 2005-223233, for example. When an orientation magnet is thus produced by arranging a plurality of small magnets so as to repel each other at outer peripheries in the circumferential direction, as discussed in Japanese Laid-Open Patent Application 2005-223233, and the surface magnetic flux density measured on an inner periphery of the cylindrical bonded magnet (the molded article) in the circumferential direction is plotted on a graph, the resulting waveform ends up being tapered.
To solve this problem, in Japanese Laid-Open Patent Application 2005-223233 the above-mentioned problem of tapering is solved, and a surface magnetic flux density that approximates a sine wave is obtained, by disposing a magnetic yoke on either side of the molding space and in between a plurality of small magnets that make up the orientation magnet.
One known method for forming an anisotropic bonded magnet is to heat and melt a bonded magnet composition composed of a magnetic material and a resin material inside a mold while performing magnetic field orientation and heat curing the material and subjecting it to injection molding (Japanese Laid-Open Patent Application 2004-158748).
Meanwhile, a known method for the injection molding of a thermosetting resin is to supply a thermosetting resin to an extruder equipped with a screw having a smooth part at the distal end portion, inducing a curing reaction in the gap between the smooth part and the cylinder, and extruding directly to the outside from the cylinder distal end (Japanese Patent Publication H6-11514).